Today, IETF uses Cloudlfare.

(Although the same old IETF IP addresses can still be accessed directly, e.g., 4.31.198.44.)

I have always used a one line script to fetch the text versions of RFCs. It has always been the fastest. In the early days all RFCs were plain text, no HTML.

Something like below except I use custom programs for HTTP generation, chunked transfer decoding and ESNI-enabled openssl. Also they allow RFC2616 HTTP pipelining so I can retrieve multiple RFCs with one TCP connection.

   test $# = 1||exec echo usage: $0 rfc-no\|draft-id
   printf "GET /rfc/rfc$1.txt HTTP/1.0\r\nHost: www.ietf.org\r\nConnection: close\r\n\r\n"|openssl s_client -tls1_3 -4 -servername www.ietf.org -ign_eof -connect 104.16.249.249:443|less

(104.16.0.0./12 is one of many Cloudflare blocks that can be used.)